Session Initiation Protocol (SIP) is an application layer signaling and control protocol for establishing and managing delivery of Internet Protocol (IP)-based telephony services in a packet-based network. SIP provides user authentication, redirect and registration services, and can be used to support a variety of telephony services such as audio or video conferencing, text messaging, interactive gaming, and call forwarding. The SIP protocol is described in Request for Comment (RFC) 3261, published in 2002, and prepared by the Internet Engineering Task Force (IETF).
SIP provides several functions for the setup, modification, and termination of sessions. In particular, SIP provides a system of rules for creating, modifying, and terminating sessions over the Internet. SIP is based on an HTTP-like request and response transaction model. Each transaction consists of a request that invokes a particular function and at least one response. SIP can operate over any transport protocol. In other words, the details of data exchanged within a session, e.g., the coder-decoder (codec) used in the session, are not controlled by SIP. Instead, SIP is compatible with other protocols to build a multimedia architecture that can provide complete services to end users.